1. Field of the Invention
The present invention relates generally to magnetic storage devices and particularly to hard disk drives having magnetic heads including pinned magnetic layers within a read head sensor.
2. Description of the Prior Art
A computer disk drive stores and retrieves data by positioning a magnetic read/write head over a rotating magnetic data storage disk. The head, or heads, which are typically arranged in stacks, read from or write data to concentric data tracks defined on surface of the disks which are also typically arranged in stacks. The goal in recent years is to increase the amount of data that can be stored on each hard disk. If data tracks can be made narrower, more tracks will fit on a disk surface, and more data can be stored on a given disk. The width of the tracks depends on the width of the read/write head used, and in recent years, track widths have decreased as the size of read/write heads has become progressively smaller. This decrease in track width has allowed for dramatic increases in the areal density data storage density of disks.
For reading the magnetic data stored on the disks, a sensor called a spin valve is commonly used which includes one or more ferromagnetic pinned layers and a ferromagnetic free layer. The first pinned layer is generally fabricated on an antiferromagnetic (AFM) pinning layer which fixes the magnetic moment of the pinned layer at an angle of 90 degrees to the air bearing surface (ABS). In this type of MR sensor, the resistance of a layered magnetic sensor varies due to both spin-depending transfer of conduction electrons between the pinned magnetic layers and the free layer through a non-magnetic spacer layer, and spin-depending scattering at the interfaces between the layers accompanying the transfer of conduction electrons. The in-plane resistance between the pinned ferromagnetic layers and the free layer, which are separated by the non-magnetic spacer layer varies in proportion to the cosine of the angle between the magnetization in the free and pinned layers.
In ferromagnetic materials, scattering of electrons depends on the spin of the carriers. Resistivity is proportional to the scattering of electrons. Electrons with spins parallel to the magnetization direction experience very little scattering and hence provide a low-resistance path. If magnetization of part of this layer structure is pinned and the magnetization of the free layer is gradually rotated from a parallel to an anti-parallel direction, the resistance of the structure increases in proportion to the cosine of the angle of magnetizations of the layers. This change in resistance is thus used to read the data bits on the disk.
The sensitivity of a magnetoresistive sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance to maximum resistance and R is the resistance of the tunnel junction sensor at minimum resistance.
As referred to above, it is common practice in the industry to pin the pinned layer by using a layer of anti-ferromagnetic (AFM) material, but in recent years there have been structures developed which are “self-pinned”, such that the sensor can dispense with the AFM layer.
Since it is the angle of magnetization between the free and the pinned layers that enables the data to be read, and since the free layer magnetization must be free to change direction with the magnetic orientation of the data-bits, it is the pinned layers that provide the steady foundation for the magnetization angle. If the pinned layer orientation changes from its correct, fixed direction, the angle between the magnetization directions in the pinned layers and the free layer changes, causing the output characteristics of the read head to deteriorate. It is of crucial importance therefore that the magnetization of the pinned layers remain stable and unchanging.
Weakness of pinning in the pinned layer is thus of concern in the manufacturing of disk drive and disk drive components. The factors which cause the strength of the pinned layer to degrade are not completely understood, but it is thought that such factors as mechanical stress caused by head/disk interaction, and electrical stress caused by electrical transients or by temperature fluctuations may contribute to this weakness. At its extreme, there the read head signal can flip its sign (positive to negative or vice versa). This is becoming a growing problem in read sensors in which the elements are becoming so miniaturized that the superparamagnetic limit for magnetic materials is being approached.
At present, there is no method for testing the strength of pinning of the pinned layers which is used for mass production. The strength of the pinning is a vital factor in the overall performance and reliability of the read head, and thus, of the disk drive as a whole.
Thus, there remains a great need for a method that can test the strength of pinning of the pinned layers at an early stage in the manufacturing process, so that weak components can be screened out, and the overall reliability of the read heads and thus of the entire hard disk drive may be enhanced.